Polymeric material and its use in multilayer products

ABSTRACT

The invention relates to a multilayer product for the production of packaging, the product comprising a carrier web (1), on one face of which there is disposed a tight multilayer coating (2) which includes a moisture-proof layer (b) and a gas-tight layer (a). The gas-tight layer is made up of a blend of an ethylene/vinyl alcohol copolymer and a polar ethylene polymer. The moisture-proof layer is polyolefin, preferably a blend of polyethylene and a polar ethylene polymer.

The invention relates to a packaging material which has good barrierproperties vis-a-vis gases and moisture. The invention further relatesto a polymeric material which is used as a gastight layer in suchpackages, and further to multilayer products in which said polymer isused.

Liquid foodstuffs, such as juices, which are sensitive to oxidation, arein general packaged in sales packages made from laminated paperboard.Through selection of the packaging material, efforts are made to providegood chemical and mechanical protection for the product, and thematerial has to be sufficiently firm in shape to be easy to handle.

There is in general use a packaging material the structural part ofwhich consists of paperboard or paper which is sufficiently firm but,nevertheless, flexible so that a package of a desired shape can beformed therefrom. To protect the structural material from liquid andother moisture, it is coated with plastic, usually polyethylene.However, the oxygen permeability of polyethylene and paperboard is high,and therefore the product would spoil very rapidly if a gas-tight layerwere not also added to the package. The gas-tight layer conventionallyused has been aluminum foil bonded to the paperboard. Anotheralternative has been a gas barrier polymer such as ethylene/vinylalcohol copolymer (EVOH). The purpose of the gas-tight layer is toprevent the access of oxygen into the product and thus, for example, tomaintain high the level of vitamin C in the product.

For some time, attempts have been made to eliminate the aluminum foil,for example for reasons of environmental protection, but also because ofits poor handlability. Furthermore, the aluminum layer is easily broken.

Finnish Patent F189567 discloses a barrier structure in whichethylene/vinyl alcohol copolymer is used. Such structures, in the sameway as when aluminum is used, usually contain, in addition to thecarrier web (polyethylene+paperboard), at least five layers:polyethylene/paperboard/polyethylene/adhesion plastic/gas-tight EVOHpolymer/adhesion plastic/polyethylene. The multilayer structure, ofcourse, also increases the price of the package and renders itsmanufacture complicated. Furthermore, the gas-tight polymer in itself isexpensive.

EP patent 423511 sought a solution in which lower amounts of EVOH wouldsuffice. Therein a blend of EVOH and polyethylene is used, in which theamount of polyethlene is at maximum 80%, but preferably at maximum 40%.The EVOH/PE layer on the inside of the package mainly serves as anaroma-proof layer and, if good gas-tightness (oxygen-tightness) is alsodesired, a second EVOH/PE layer or some other oxygen-tight layer isadded, with adhesion plastic layers on its both sides. In this case thenumber of layers is four in addition to the carrier web.

An object of the present invention is to provide a multilayer product bymeans of which the disadvantages of the prior art can be eliminated.When polymers according to the invention are used, the number of layerscan be reduced from the conventional number, which is usually five inaddition to the carrier web, since a separate adhesion plastic layer isnot needed. It has been observed, surprisingly, that, if a blend of acommonly known gas-tight polymer, i.e. ethylene/vinyl alcohol copolymer(EVOH), and of a polar ethylene polymer is used in the gas-tight layer,this layer having good adhesion properties and being heat-sealable, theuse of the adhesion plastic layer can be abandoned.

In the system according to the invention, the gas-tight layer contains apolar ethylene polymer in an amount of 5-60%, preferably 15-40%. Thepolar polymer may be a copolymer of ethylene and acrylic ester, such asethylene/ethyl acrylate copolymer (E/EA), ethylene/butyl acrylatecopolymer (E/BA), ethylene/methyl acrylate copolymer (E/MA),ethylene/vinyl acetate copolymer (E/VA), or a copolymer of ethylene and(met)acrylic acid (E/EA, E/MAA). The polar polymer may also be ananhydride- or acid- modified polyolefin, in which case the acid(anhydride) used may be, for example, maleic, fumaric or itaconic acid(anhydride). Furthermore, the polar polymer may also be a terpolymer,such as a terpolymer of ethylene, methyl acrylic acid and butyl acrylicacid (E/MAA/BA) or a copolymer of ethylene, acrylic ester and maleicanhydride. The basis weight of gas-tight polymer is typically 1-20 g/m²,preferably 2-10 g/m².

Many advantages can be gained by using, instead of ethylene/vinylalcohol copolymer, a blend thereof and a polar ethylene polymer. Thepolymer blend also serves as a good oxygen barrier layer when it isinside a coextruded structure, in the same way as does EVOH alone. Whenthe layer made of the polymer blend is on the inside surface of thepackage structure, a good aroma-proofing is also achieved. As comparedwith a mere ethylene/vinyl alcohol polymer, a lower blistering tendencyand a higher resistance to heat and moisture are achieved. Since onecomponent in the polymer blend is polar, as a consequence bettersealability properties, good hot-tack properties, and better adhesionproperties are achieved than by using a pure polymer. As a result, theuse of a separate adhesion plastic layer can be abandoned. Furthermore,the processibility of the polymer blend is better, and smaller layerthicknesses can be used.

Polyolefin, preferably polyethylene, which may be any polyethylene (LD,HD, LLD, VLD, etc.), is used as the moisture-proof layer. Preferably apolar polymer selected from the same group as the polymers in thegas-tight polymer blend is added to the polyethylene, in an amount of2-50%, preferably 20-30%. By this addition, good adhesion of the layersto each other is ensured, as is good seal strength at lower sealingtemperatures. Furthermore, the blistering tendency is lower. Environmentstress crack resistance (ESCR), water vapor transmission rate (WVTR) andfrictions are easier to control. Resistance to wear is also better,especially if high-density (HD) or linear (LLD) polyethylene is used,and consequently, plastic dust will not be formed on rapidpackage-manufacturing lines and the blanks will travel smoothly on thelines.

The polar polymer can be mixed with the ethylene/vinyl alcohol polymerand, respectively, with the polyolefin by any conventional mixingtechnique. Either melt mixing or dry mixing can be used.

The carrier web may be any web which can be coated, but a combination ofpaperboard and polyethylene is highly suitable for packages for liquids.The other layers are typically bonded to the carrier web by coextrusioncoating.

The number of layers placed on one side of the carrier web, and theirmutual order, can be varied according to the application and inparticular according to the requirements set by the product to bepackaged. With smaller numbers of layers, the price of the multilayerstructure, i.e. the packaging material, will be considerably lower.According to the present invention, 2-layer coextrusion instead of theprevious 5-layer coextrusion will well suffice for many applications.Owing to the small number of layers, pulpability will be considerablyimproved, and blistering tendency in the heat-sealing step is nil. Theimproved pulpability is due above all to the fact that an EVOH/PE layerbonded directly to paperboard detaches from the paperboard more easilythan does an adhesion plastic layer. In consequence to easier pulping,recycling of the material will be simpler.

Depending on the application, the gas-tight layer (a) and themoisture-proof layer (b) may be in different orders on the carrier web:a-b, a-a- a-b-a. The order depends on the product to be packaged, i.e.on whether protection is needed specifically against the permeability ofmoisture, oxygen, aromatic substances, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of the prior art which does not have a polarethylene copolymer blended in the gas-tight layer.

FIG. 2-3 are examples of structures according to the invention in whichthe gas-tight layer comprises a blend of polar ethylene polymer andethylene/vinyl alcohol copolymer.

The invention is described in greater detail with the help of thefollowing examples.

EXAMPLE 1

A gas-tight plastic was prepared by blending an amount of 80% ofethylene/vinyl alcohol copolymer, EVOH (44% ethylene), manufacturer DuPont and trade name Selar OH 4416, with an amount of 20% ofethylene/ethyl acrylate copolymer, EEA, manufacturer Borealis and tradename NCPE 5860. The mixing was performed in a Berstoff twin screwextruder (.O slashed.=40 mm).

A carrier web (15 g/m² PE-LD and 300 g/m² paperboard) was coextrusioncoated on a Beloit pilot line with a double-layer coating in which thegas-tight layer consisted of 10 g/m² (EVOH+EEA) and the moisture-prooflayer consisted of 30 g/m² low-density polyethylene, PE-LD, manufacturerBorealis Polymers Oy and trade name NCPE7518. There were thus only twolayers in addition to the carrier web. The oxygen permeability of thematerial was 6 cm³ /m² per 24 h at a temperature of 23° C. and arelative humidity of 50%.

the oxygen permeability of a corresponding conventional 5-layerstructure was 4 cm³ /m² per 24 h. The layers of the conventionalstructure were: carrier web 15 g/m² PE-LD and 300 g/m² cardboard, and ontop of it a 5-layer coextrusion coating consisting of 20 g/m² PE-LD, 6g/m² adhesion polymer, 10 g/m² EVOH, adhesion plastic 6 g/m², and PE-LD20 g/m² (FIG. 1). The adhesion polymer used was polyethylene modifiedwith maleic anhydride, trade name NCPE 0470, manufacturer BorealisPolymers Oy.

The saving of material in the structure according to the invention isconsiderable as compared with traditional structures, and thegas-tightness is of substantially the same level.

The structure according to the invention is shown in FIG. 2.

EXAMPLE 2

A gas-tight plastic was prepared by mixing an amount of 80% ofethylene/vinyl alcohol copolymer, EVOH (38% ethylene), manufacturerKuraray and trade name EVAL 101H, with an amount of 20% of polyethylenemodified with maleic acid, manufacturer Borealis Polymers Oy and tradename NCPE 0470. The mixing was carried out in the same manner as inExample 1.

A carrier web was coextrusion coated in the same manner as in Example 1,but the thickness of the gas-tight layer was only 6 g/m². Thepermeability to oxygen was 3 cm³ /m² per 24 h, 23° C., 50% relativehumidity.

In multilayer packaging materials there has often been a problem ofblistering during sealing. The blistering tendency of this productaccording to Example 2 was 50% less than that of the control productaccording to FIG. 1.

EXAMPLE 3

To maximize internal adhesion and sealability, modified polyethylene wasused also as the polyethylene layer (PE-LD). An amount of 25% ofanhydride-modified polyethylene, NCPE 0470, and an amount of 75% ofPE-LD, NCPE 7518, were mixed. The thicknesses of the layer were the sameas in Example 2. As a consequence, the adhesion to the gas-tight layerand the heat-sealing properties were further improved. Blisteringtendency also decreased further.

EXAMPLE 4

In order to achieve maximal aroma-proofing, impermeability andflavor-tightness of ethereal oils, a structure according to Example 4was prepared, but a surface layer similar to the gas-tight layer wasadded to it (FIG. 3). Such a structure has very good properties but,nevertheless, in addition to the carrier web it has only three layersinstead of the traditional five.

EXAMPLE 5

In order to improve further the sealing properties and watertightness ofthe layer coming against the product, the mixing proportions of thegas-tight layer (a) were optimized. A gas-tight plastic was prepared bymixing an amount of 60% of ethylene/vinyl alcohol copolymer,manufacturer Du Pont and trade name SELAR OH 4416, and an amount of 40%of polyethylene modified with maleic acid, manufacturer BorealisPolymers Oy and trade name NCPE 0470. The mixing was carried out as inExample 1.

A film 40 μm thick was prepared of the blend. The permeability of thefilm to oxygen was 1.3 cm³ /m² per 24 h, 23° C., 50% relative humidity.The oxygen permeability of a film of the same thickness made of pureethylene/vinyl alcohol copolymer was 0.3 cm³ /m² per 24 h, 23° C., 50%relative humidity. If a gas-tight blend such as this, in which the ratioof the polymers is 60/40, is used in a layered structure a-b-a, i.e.there are two gas-tight layers, good gas-tightness and aroma-proofingproperties are achieved, and the processibility of the gas-tight blendis considerably better than that of pure ethylene/vinyl alcoholcopolymer.

The blistering tendency was very low.

EXAMPLE 6

By the use of polymer blends according to the invention, the meltstrength of the gas-tight layer can be improved substantially and theso-called blistering tendency of the structure is reduced. Blisteringtendency was measured by using a device specially developed for thepurpose, in which hot-air blowing is used.

The sample to be tested is attached to a holder, which makes aconstant-velocity reciprocal movement under hot-air blowing. Thedistance from the blower to the sample is 7 mm, and the temperature ofthe hot air is 350° C. The blowing causes melting of the plastic. Thesample will resist without melting under hot air for a number ofreciprocal movements which is the greater the better the resistance ofthe sample to blistering.

The test device clearly correlates with results in practice in packagingmachines in which hot air is also used for sealing.

Table 1 shows the results of the blistering test for the controlmaterial according to FIG. 1 and for materials according to Examples 3and 5. The number indicates the number of reciprocal swings. Theblistering tendency of the structures according to the invention isconsiderably lower than that of the control structure.

                  TABLE 1                                                         ______________________________________                                        Blistering tests in a hot-air blowing device                                    number of                                                                     swings                                                                        Product 1 1,5 2 2,5 3 3,5 4 4,5 5                                           ______________________________________                                        Control, 5-layered                                                                        U     F      F   F    M   M    --  --  --                           Ex. 3, 2-layered U U U U U V F F M                                            Ex. 5, 3-layered U U U U U U U F F                                          ______________________________________                                         U = unbroken                                                                  V = very few holes                                                            F = few holes                                                                 M = many holes                                                           

As can be seen from the above examples, by the use of gas-tight andmoisture-proof polymer blends according to the invention, considerablesavings of material can be achieved and simpler manufacturing techniquescan be used and, nevertheless, the permeability and other propertiesobtained for the package will be at least as good as those achieved withconventional alternatives. The processibility of the EVOH/PE blend isbetter than that of pure EVOH, which is due to the better melt strengthof the blend. Consequently, a thinner gas-tight layer than conventionalwill suffice.

We claim:
 1. A multilayer product for the production of liquid packagingcomprising:(1) a carrier web, wherein said carrier web comprises paperboard having an inner and an outer surface; and (2) a multilayercoextruded coating disposed on the inner surface paper board, whereinsaid multilayer coating includes in any order disposed one on top of theother:(a) a gas-tight layer comprising a blend of an ethylene/vinylalcohol copolymer and a polar ethylene polymer, wherein said gas tightlayer comprises said polar polymer in an amount of 5-60% by weight, and(b) a moisture-proof layer comprising polyethylene.
 2. The multilayerproduct according to claim 1, wherein the polar polymer in the gas-tightlayer (a) is a copolymer of ethylene and acrylic ester, a copolymer ofethylene and vinyl acetate, or a copolymer of ethylene and (meth)acrylic acid.
 3. The multilayer product according to claim 2, whereinthe acrylic ester is a selected from the group consisting of ethylacrylate, butyl acrylate and methyl acrylate.
 4. The multilayer productaccording to claim 1, wherein the polar polymer in the gas-tight layeris a polyolefin modified with a carboxylic acid or carboxylic anhydride.5. The multilayer product according to claim 4, wherein the polarpolymer in the gas-tight layer is a polyethylene modified with maleicanhydride.
 6. The multilayer product according to claim 1, wherein thepolar polymer in the gas-tight layer is a copolymer of ethylene, acrylicester and carboxylic acid or anhydride.
 7. The multilayer productaccording to claim 1, the ethylene content in the ethylene/vinyl alcoholcopolymer of the gas-tight layer is 20-50% by weight.
 8. The multilayerproduct according to claim 1, wherein the moisture-proof layer (b)comprises 50-98% low density polyethylene and 50-2% a polar polymerselected from the group consisting of copolymers of ethylene and acrylicester, copolymers of ethylene and vinyl acetate and copolymers ofethylene and (meth) acrylic acid.
 9. The multilayer product according toclaim 1, wherein said carrier web further comprises an plastic layer,said plastic layer being coated on the outer surface of said paperboard.10. The multilayer product according to claim 6, wherein the polarpolymer in the gas-tight layer is ethylene/butyl acrylate/(meth) acrylicacid terpolymer.
 11. The multilayer product according to claim 9,wherein the amount of polar polymer in said moisture-proof layer is20-30% by weight.